Safe and clean drinking-water is a basic need for human development, health, and well-being. As global industrialization and economic development continue growing, the concerns associated with water contamination are becoming more serious and urgent to be addressed. The increasing consumption of contaminated water for human is also arising more and more health-related public concerns. Therefore, the need for improving water purification technology continues to grow dramatically in the U.S. and abroad.
Generally, the contaminants in the water can be categorized into chemical contaminants and biological contaminants. As water pollution is increasing in occurrence, the potential health and safety issues associated with the chemical contaminants in the water is becoming a more prominent global concern. Some examples of the chemical contaminants include toxic anions (fluoride, arsenite, arsenate, nitrate, chromate, selenite, selenate, etc.); metals; heavy metals (lead, mercury, cadmium, zinc, copper, chromium, etc.); synthetic or natural organic matters; etc. It is well known that most of the heavy metals are toxic to human beings and should be removed from drinking water.
Additionally, many water treatment applications are required to meet specific regulatory requirements pertaining to the removal of these species prior to discharge. These regulations are subject to change as a result of scientific findings as well as with improved detection/analytical techniques. For example, in 2007, the NSF Intl. Drinking Water Treatment Unit Joint Committee revised the NSF/ANSI Standard 53 protocol for pH 8.5 lead reduction based on research into the nature of lead particles. The new protocol specifies a size range for the colloidal or fine particle portion (between 0.1-1.2 micron) which was undefined previously. This change did not pose a significant problem for pressurized filters (e.g. carbon blocks) but did introduce additional challenges for low pressure (less than 30 psi) and gravity flow filters, which have difficulty with colloidal materials
Gravity flow or low pressure flow filtration systems are well known in the art, because of their generally lower cost and user convenience. Such systems include pour-through carafes, water coolers and refrigerator water tanks, which have been developed by The Clorox Company®, Culligan®, Rubbermaid®, and Glacier Pure®, et al. Typically, these systems are filled with tap water from municipal supplies or rural wells, as the user wishes to remove chlorine and/or lead or other chemical contaminants, or to generally improve the chemical safety of the water and the taste/odor of the water. The marketing need of these devices is continuing to grow quickly, especially in view of the emphasis on healthier and safer drinking water, and further in view of the expense and inconvenience of purchasing bottled water.
Most of the gravity-fed or low pressure flow filters utilize a combination of granular filtration media, such as granular-activated carbon (GAC) and ion exchange resin (IER). These devices have been proven effective in removing contaminants such as organics, copper, mercury, cadmium, zinc, and residual chlorine, etc., within compliance of regulatory standards. Commercially, these filtration devices typically feature relatively small, disposable and replaceable filter cartridges that are inserted into the water purification devices and used for several weeks of normal use. However one problem associated with filters containing a mixture of granular activated carbon and ion exchange resin is that they have limited contaminant removal capability. When large granules are packed together, large interstitial voids can form between the granules, which results in effective pore sizes which are larger than colloidal particles. These particles, like the ones specified by the NSF/ANSI 53 protocol, could pass through these voids and into the effluent, thus may fail to meet regulatory standards.
Granular activated carbon, with or without binder, and with or without various other additives such as lead scavengers, has been well developed and broadly used as a filtration media in water purification filters for many years. The granular activated carbon is typically loaded into a compartment inside a filter housing to act as a filter or a carbon “bed”. The housing and internals are designed to contain the loose granules in place in the compartment, to distribute water to the inlet of the bed, and collect the water at the outlet of the bed. Generally, a bed of GAC, with optional other granular media or additives, is the typical media composition of choice for low pressure or gravity flow applications, because of the relatively low pressure drop through the bed of granules than other media.
Good water flow rate through the filter is another primary concern in a low pressure or a gravity flow water system such as a water pitcher device, water cooler device, or the like because this affects how quickly filtered water from a freshly-water-filled device may be used to satisfy the consumer's expectation. That is why the granular filtration media mixture is frequently selected to fill in those types of filters.
Overall, the ideal filter for the gravity-fed or low pressure device provides high efficiency at contaminant removal and high flow rate. The existing gravity flow or low pressure flow filters can generally achieve a good flow rate, however, as mentioned previously, they also have some limited contaminant removal capability to removal particulate contaminants from the water source. Therefore, the existing gravity flow and low pressure flow granular filtration media mixture needs to be improved to achieve higher contaminant removal efficacy, specifically with regards to colloidal and suspended particles.
Knipmeyer, in U.S. Pat. No. 8,167,141, discloses a gravity-fed carbon block water filter comprising an activated carbon and a lead scavenger, which could deliver a final effluent water containing less than 10 ppb after 151 liters of source water filtration to meet the revised NSF standard of lead removal claim. However, when carbon blocks designed for pressurized systems are applied to gravity flow systems, they often add more cost and fail to produce the desired flow rates consistently over time.
U.S. Pat. No. 8,002,990, issued to Schroeder on Aug. 23, 2011, discloses a filter using fibrillated nanofiber-loaded fine powders of filter media, such as ion exchange resin, to remove soluble and insoluble particles from a fluid. However, when the media is used in a gravity-fed or a low pressure filtration system, the flow rate will be significantly reduced in comparison with the granular filtration media. Furthermore, compared with the present commercially available gravity-fed or low pressure-fed filters filled with granular filtration media the cost will also be more greater.
Koslow, in U.S. Pat. Nos. 6,872,311; 6,913,154, discloses the use of nanofibers to improve filtration efficiency. These patents teach a fibrillated physical process that can enhance the performance of existing standard filter media such as cellulose fiber, and further teach a method of making an improved air filter medium incorporated with nanofibers. However, this invention does not teach how to improve the existing granular filtration media for the purpose of removal of particulates from a contaminated water by using nanofibers.
Halbfoster, in U.S. Pat. No. 4,190,532, describes a filter material composition comprising a mixture of ion exchange resin particles and cellulose filter aid for use in removing suspended and colloidal particles, such as silica or iron oxide, from water. This invention has been commercially practiced in the high quality water supply process for a long time, however the particle removal efficacy still needs to be improved.
It is believed that there is a need to improve existing granular media (and combinations) for use in gravity flow and low pressure filters such that adequate flow rates are achieved while maintaining high contaminant removal. Specifically, there is a need for media that can remove colloidal and suspended particles from water, such as particulate lead as specified in NSF/ANSI 53.